With chip supply constraints, many manufacturers have turned to reclaimed or second-hand ICs to keep production lines running. However, assemblies built with recycled BGA devices frequently exhibit soldering defects. This case documents a high open-solder rate observed on a CPU BGA when using reclaimed material, the investigation performed across stencil printing, placement, reflow, and component condition, and the corrective actions that resolved the issue.
Problem Description
On the first article, X-ray inspection at U1 (a CPU) revealed multiple open solder joints on BGA balls. The lot used reclaimed devices. After a series of process adjustments and validations, 13 boards were built; 10 exhibited CPU opens, a defect rate of 76.9%. Key product and package information is as follows:
- PCB size: 170 × 170 × 1.6 mm
- CPU size: 42 × 24 × 1.3 mm
- CPU BGA pitch: 0.65 mm
- Solder ball count: 1356
- Lot built: 13 pcs; boards with CPU open solder joints: 10 pcs
Representative X-ray images of the CPU BGA with open solder joints are shown below.
Root Cause Analysis
1. Verification of Solder Paste Printing and Placement
A customer-supplied laser stencil was used with a thickness of 0.13 mm and circular apertures of 0.34 mm diameter.
Solder paste deposits released cleanly with good shape and volume. No insufficiencies or missing prints were observed.
Pick-and-place accuracy was verified by inspection of the placed CPU; no placement offset or skew was found.
Interim conclusion: The stencil design and solder paste printing process were appropriate for the BGA pitch. Printing quality was good, and placement alignment was normal. These processes were not the primary source of the open joints.
2. Reflow Profile Confirmation
The reflow oven profile was validated against the solder alloy process window; measured parameters met soldering requirements, with no anomalies recorded during the trials.
3. Microscopic Inspection of Solder Ball Surfaces
Microscope inspection of the CPU solder ball array did not reveal obvious contamination, oxidation, or surface residue that would inhibit wetting.
Interim conclusion: The solder ball surfaces did not show significant oxidation or contamination, so surface chemistry alone did not explain the open joints.
4. X-ray Observations and Warpage
X-ray revealed a pronounced "lift" or stretch on the corner-column solder joints after reflow, while solder joints near the center showed normal collapse. This pattern is characteristic of BGA package warpage, where the component corners lift away from the PCB during thermal cycling.
On the second board after reflow, the BGA was observed to be severely warped with visible corner lift. This mechanically explains the X-ray indications: as the BGA bows, a gap opens between some solder balls and the paste deposits at the PCB pads, preventing proper wetting and coalescence and resulting in open joints.
5. Cause Synthesis
Process checks confirmed the following: solder paste printing was within expectation with no insufficiency or missed prints; placement was accurate; the reflow profile was appropriate; and solder ball surfaces did not show evident oxidation or contamination. X-ray consistently showed corner-joint lift with normal joints in the center, indicating a mechanical deformation issue with either the component or the PCB during reflow.
Further observation confirmed that the CPU package itself exhibited significant warpage. During soldering, the warped package created a gap between the BGA balls and the solder paste on the PCB pads at the lifted corners, which led to open solder joints.
Contributing factor: the device was reclaimed ("old material"), and its body thickness was only 0.8 mm. Prior to this build, the component had already experienced four high-temperature exposures (initial new-product assembly, desoldering from the original PCB, solder removal, and reballing). The present assembly added a fifth high-temperature cycle, exceeding the common guideline of three reflow exposures. Each additional exposure increases the risk of package warpage due to CTE mismatch, substrate softening, and residual stresses. The resulting deformation during the final reflow created gaps at the solder interfaces, producing open joints at the corners. The schematic below illustrates the gap mechanism.
Corrective Actions
To address the open-solder failures, the following mitigations were defined and trialed:
- Light abrasion of BGA solder balls using fine abrasive cloth to refresh surfaces.
- Adjust the reflow profile to an RTS (Ramp-to-Spike) profile for production.
- Use a press block (hold-down weight) during reflow to mechanically suppress BGA package warpage.
Additional inspection guidance:
- At the SMT stage, use 2D X-ray mode for inspection rather than 2.5D mode.
Corrective Action Implementation and Results
1. Press Block Application
The press block was installed to apply uniform, controlled pressure over the BGA area during reflow, reducing corner lift and maintaining contact between solder balls and paste.
2. Comparative Results
Per customer request, the team validated both BGA solder ball surface treatment and the RTS profile. The comparative results are summarized below.
Sanding and cleaning the solder balls did not yield a noticeable improvement. In contrast, using the press block eliminated the open-solder defect rate (100% reduction of opens in the trial). However, two instances of solder bridging occurred. Subsequent analysis identified the press block weight as excessive, contributing to bridging. After optimizing and reducing the block weight, bridging defects were eliminated.
Conclusions and Practical Guidance
For assemblies built with reclaimed BGA devices, repeated prior thermal exposures and thinner package structures significantly increase the risk of package warpage during reflow. Warpage induces gaps at the solder interface—especially at package corners—leading to open joints even when paste printing, placement, and reflow parameters are within process windows.
When X-ray reveals corner-joint lifting with normal collapse in the center, suspect package or PCB warpage. If process controls and surface condition are confirmed normal, mechanical mitigation can be decisive. In this case, a properly designed press block applied during reflow suppressed warpage and restored reliable joint formation. If employed, ensure the block's contact area, pressure distribution, and mass are optimized to avoid side effects such as solder bridging. Profile adjustments such as an RTS curve may be used in parallel, but mechanical hold-down proved to be the most effective control for this warpage-driven failure mode.
Finally, when planning to reuse BGA devices, track cumulative thermal cycles and consider requalification steps (including flatness checks) commensurate with the device's prior exposure history. The cost of warpage-induced defects can quickly outweigh the savings from reclaimed components if not properly managed.
